Unlike most automobiles, commercial tractor/trailer vehicles typically utilize large diesel engines requiring substantial amounts of short-term power (e.g. about 1800 amperes) to start the engine. Such large current demands result from, among other things, the requirements of the fuel injection systems of such engines. In order to satisfy the large current demands of the diesel engines, commercial tractor/trailer vehicles are typically equipped with battery power systems including a plurality of high cold-cranking-ampere (CCA) batteries connected in parallel.
Supplying high cold-cranking current is not the only requirement for battery power systems used in commercial tractor/trailer vehicles. Such systems must also provide electrical power during times when the engine is not running. Of course, while the engine is running, ample electrical energy is usually provided by the alternator to operate such electrical devices as communication equipment, radios, lights, and other creature-comfort devices including microwave ovens, electric blankets, televisions, video tape recorders, etc. However, when the engine is not running, if any electrical devices are to be powered, they must be powered by the battery power system or an alternative system.
In earlier battery power systems, 12-volt batteries were utilized to both start the engine and run any electrical appliances when the engine was not running. Such systems experienced very significant problems, in that the electrical appliances would run the batteries down to the point that they would be unable to start the engine when required. This often occurred if the electrical appliances were run for a long period of time. In order to prevent this situation from occurring, systems were developed to isolate some batteries from the electrical appliances during times when the engine is not running. Those batteries would then only be used to start the engine.
Presently, a so-called "3+1" battery power system is used to start the engine and power any electrical accessories when the engine is off. This system uses four "Group 31" 12-volt batteries which are typically enclosed in a battery box. "Group 31" refers to the size and shape of the battery which is the industry standard for commercial tractor/trailers. To guarantee sufficient cold-cranking current, three of the Group 31 12-volt batteries are connected in parallel and used only for starting the engine. The remaining battery (the "+1" battery) is used exclusively for the key-off operation of the electrical accessories. When the engine is off, the accessory battery is isolated from the starting batteries. After the engine is started, the accessory battery is switched in parallel with the starting batteries so that it can be charged by the engine's alternator.
Although isolation power systems such as the 3+1 battery power system ensure sufficient starting power by preserving the charge on the starting batteries, such systems work at the expense of providing power for electrical accessories while the engine is off. As a result, several alternatives have emerged to deal the limitations of these battery power systems. For example, a small minority of tractor/trailers are equipped with small diesel-powered generators to supply electrical energy to the appliances while the main diesel engine is off.
Where the tractor/trailer is not equipped with such generators as is the usual case, the drivers may elect to run the engine while they are resting or may choose to run the accessory battery down until it completely runs out of energy. Since running the engine is very expensive and potentially dangerous, many drivers choose to completely drain the accessory battery. Due to such repeated over-discharging, the accessory battery in a 3+1 battery power system cannot be fully recharged in many instances during normal operation of the vehicle. This causes a significant reduction of service life of the accessory battery. In fact, it is quite common for accessory batteries to need replacement every two or three months.
The fundamental problem with 3+1 battery power systems is that they fail to provide key-off power for an adequate amount of time. For example, during cold winter nights, a driver may depend on an electrical blanket to keep warm. Even if the driver decides to let the accessory battery run until it is out of energy, the driver will loose power well before he or she is ready to drive again. Typically, the accessory battery is at a low state of charge when asked to provide power, which significantly reduces the time the driver may use the blanket. In such a case, the electrical blanket will cease to work during the night. This is obviously unacceptable.